The present invention is related to a method of automatic register setting of printings in a rotary machine and to a device for working the method.
1. Background of the Invention
The method refers in particular to the automatic register setting of printings printed by plate cylinders in a rotary machine such as a heliographic, flexographic or other printing machine, printing a web such as paper, cardboard or a flexible material like, for example, polyethylene.
2. Description of the Related Art
This kind of machine usually comprises several printing units placed one after another and each printing a different color. A printing unit includes a plate cylinder and a pressing cylinder between which travels the material to be printed. The plate cylinder is either and engraved cylinder or a cylinder equipped with a printing plate reproducing the printing pattern. To obtain a perfect result, it is necessary that the various printings performed by the printing units are accurately superimposed. Each printing unit prints a register mark on the material web and the register setting of the various colors is achieved by superimposing the various register marks by acting, for example, on the lengthening or on the tension of this web by means of one or more compensating rollers arranged between the printing units. Such a method of register setting is described in patent CH 539 509. This method determines the register error at a printing station, then, from the result of this determination is calculated, according to the travelling of the web, a reference tension or lengthening of this web between the printing station concerned and the preceding one, this tension or this lengthening allowing an optimal correction of the register error. The following operation consists in comparing the reference tension or lengthening with the real tension or lengthening of the web between the printing station in question and the preceding one, in order to act on the web according to the result of this comparison in order to bring the tension or lengthening of the web to the value of the reference tension or lengthening.
This method requires of course a register mark scanning which requires the creation of a scanning gate in order to be absolutely sure to scan only the marks previously printed on the web, excluding any other printing part. Usually, the register marks are printed on a space free of any other printing, for example between two printed patterns or images, or in one of the edges of the web called xe2x80x9cbank edgexe2x80x9d. Patent CH 548 933 describes in detail a method of obtaining a scanning gate.
It is quite clear that this method of register setting will function accurately only at constant operating speed of the printing machine, i.e. in the production phase, but when starting the machine, it will first be necessary to adjust the various plate cylinders with one another. This operation is carried out manually by the machine operator or by means of a system of prepositioning. To do so, the operator will carry out a manual presetting of the plate cylinders and/or the compensating rollers in order to bring all the register marks printed by each plate cylinder in the space defined by the scanning gate. Then he will enter into the memory of the calculator of the register setting device various parameters such as the data related to the angular position of the plate cylinders, the data related to the position of the compensating rollers, as well as other information so that, when repeating the same job, it will not be necessary to start again this operation.
This presetting can also be realized by calculating the angular position of the plate cylinders according to the web length between two printing units and to the circumference of the plate cylinders which corresponds to the printing size. In this calculation, the ratio must necessarily be an integral number.
After this presetting, it may occur that the position of the register marks is somewhat out of the limits of the scanning gate. If required, a repositioning should be carried out, to bring these register marks back in this gate by an additional operation called xe2x80x9cphasingxe2x80x9d. It may even occur that, the presetting having been badly done, the marks are within the printing and that the phasing is not possible to be carried out, in this case a new setting will be necessary.
Generally in the prior art, the various stages for adjusting the cylinders and/or compensating rollers require at least the following operations: the operator will first arrange the plate cylinders in their respective position according to their point of reference. Then he will preposition the cylinders and/or compensating rollers by modifying, in that case, the web length between the printing units. He will then carry out a rough manual setting at slow run before proceeding with the phasing operation by means of an oscilloscope or automatically by means of a code. Resulting from this last stage, necessary corrections will be made in a new manual register which will imply by recurrence effect a new phasing operation followed by precise adjustments before being able to undertake the production phase.
The application of the operations just described hereinbefore involves, in most cases, a high consumption of web for bringing the machine, from the start-up phase, to a constant operating speed in the production phase.
This high consumption of web comes from several factors but principally from the fact that the presetting of the plate cylinders and/or compensating rollers is carried out visually by the machine operator and that he may make important appreciation errors in setting at once all the plate cylinders. Indeed, a bad setting may involve that the register mark is not printed in the scanning gate, in which case there is no possible phasing. Moreover, at the time of the start-up, the possibility of setting errors can be to the maximum of more or less a half printing size if the presetting is not correct, with the risk that the register mark is printed in the area corresponding to the printed image so that any detection of the register mark and hence any phasing is impossible. Numerous other problems affect the setting operation of the plate cylinders which depends in particular on the precision in inserting the cylinders, on the variety of the substrates used, on the different tension levels of the web between the printing units or the sometimes variable webs. These parameters, to quote only those, are often difficult to control. They cause inaccuracies which have repercussions in series and intensify downstream between the various printing units in order to finally exceed the tolerable values specified for the automatic register setting.
To face competition and to increase the flexibility of the services offered in the field of industrial graphic arts, the production series became increasingly smaller thus causing frequent job changes. These changes cause much waste paper and lost time in the preparation phase, which will finally reflect on the cost prices of the finished products.
The aim of the present invention is to overcome the aforecited disadvantages and particularly to reduce on a large scale the time and the consumption of web when setting a rotary printing machine.
The invention, such as described hereafter, allows a simple and automatic register setting which authorizes a maximal initial register error. The method is the same for the preparation of a new job or about an old job. The data relating to the web and to the length between the units are not necessary any more. The phasing and the register setting are done with only one color which avoids any misunderstanding or overlapping of the marks resulting from several overprintings. The corrections of the register errors can be settled very quickly by moving the compensating rollers at high speed, typically about 15 mm/s, without causing any change in the web tension. A possible error of the web path in its travelling around the numerous idling rollers used between the printing units, for example for the web drying, modifies without consequence the length of the web path. According to the invention, the register setting method is not affected by an operation which requires the change of a plate cylinder or a pressing cylinder. Finally, owing to the method of this invention, the printing units must not necessarily be adjacent to one another. It may occur that, for example for maintenance, any printing unit is set out of function in the middle of the rotary printing machine. This setting out of function would not disturb in any way the automatic register setting method.